1. Field of the Invention
The present invention relates to a thermal conduction vacuum gauge, and more particularly, to a vacuum gauge using a Peltier tip.
2. Description of the Related Art
According to the definition of the American Vacuum Society(1958) the term vacuum refers to a given space filled with gas at pressure below atmospheric pressure, i.e., having a density of molecules less than about 2.5xc3x971019 molecules/cm3. Due to the advancement of vacuum technology, available vacuum range is between 10xe2x88x9213 Torr and atmospheric pressure(760 Torr). Pressure is measured with vacuum gauges operating on different principles depending on the pressure range being measured. A thermal conduction vacuum gauge is primarily used to measure the pressure in the range from 10xe2x88x924 Torr to atmospheric pressure.
A filament heated in a vacuum loses energy due to thermal conduction through a metallic wire, gaseous thermal conduction, and thermal radiation. A thermal conduction vacuum gauge operates on the principle that gaseous thermal conduction loss from the filament and, therefore, the filament temperature varies as a function of pressure. Thermal conduction vacuum gauges currently in use may include Pirani vacuum gauges, thermocouple vacuum gauges, and convection vacuum gauges. In the Pirani vacuum gauge the temperature of the filament is found from its resistance while in the thermocouple vacuum gauge a junction of dissimilar metals provides a temperature-dependent output voltage. From the fundamental point of view, the Pirani and thermocouple gauges differ only in the means of observing the filament temperature.
While the above thermal conduction vacuum gauges typically exhibit high sensitivity in the range from 10xe2x88x922 Torr to 100 Torr, they have significantly lower sensitivity below 10xe2x88x922 Torr because radiation losses, which are independent of pressure, become dominant in this pressure range. Furthermore, above 100 Torr, the mean free path of gas molecules becomes so short as to form a hot air sheath around the filament, thus preventing further gaseous conduction and severely decreasing the sensitivity of the gauges. A convection vacuum gauge is a Pirani gauge with the additional capability (and thus additional complication and cost) of inducing a forced convection in order to improve the sensitivity above 100 Torr.
To provide a better solution to the above problems than the existing gauges do, the present invention offers a new thermal conduction vacuum gauge capable of operating at high sensitivity over a wide range of pressure and having a simple structure and a micron-size.
Specifically, to achieve the above objects, the present invention offers a vacuum gauge using a Peltier tip including: a signal generator that drives an electric current into the Peltier tip; an ammeter connected to the signal generator for measuring the current; a bridge circuit, which is connected to the ammeter, consisting of thermocouple wires and a variable resistor, wherein one junction of dissimilar metallic wires is made in the form of a Peltier tip, inserted into a vacuum chamber; and a lock-in amplifier connected to the two symmetrical points of the bridge circuit for detecting a voltage signal due to the temperature oscillation at the Peltier tip and thus measuring the pressure in the vacuum chamber.
The vacuum gauge according to the present invention uses only one junction of dissimilar thermocouple wires as a heater and sensor, i.e., the junction works as a heat source and temperature sensor simultaneously, thereby reducing the overall size of the gauge and increasing the sensitivity thereof.